is representing the squared charges of the thermalized quark-gluon phase (shown as reference in Fig. REF  as “Born” approximation).
The thermal average has been taken by working out the respective phase-space integrals of the initial and final states of all partons in the fireball, and taking into account Bose-Einstein and Fermi-Dirac distribution functions as well as Pauli-blocking factors for the parton final states [1]}.
The microscopic properties of the QGP enter the emissivity of the QGP just through the number \(R_{\mathrm {part.}}\) .
For \(M/T \gtrsim 5\) , corresponding to the IMR, the functional form given by Eq. REF  agrees within 5% with the description of black body radiation.
Thus, the detection of thermal radiation from the QGP provides a direct measurement of the mean temperature of the emitting medium.
The mean is taken over the 4-volume, i.e. the respective volume \(V(T)\)  in which the matter is of partonic character.
Due to the comparatively high temperature of the QGP phase this radiation of partonic origin dominates the thermal radiation in the IMR.
